Oscilloscopes, spectrum analyzers, heart monitors and other electronic instruments often use an analog vector cathode-ray tube (CRT) to display waveform information. In the typical analog vector CRT, an electron beam sweeps at a fixed speed in one direction while being deflected by an analog signal at a variable speed in a perpendicular direction. Analog CRTs have an inherent intensity variation as a function of the velocity of the electron beam across the face of the display (slew rate) and the intensity build-up and decay times (persistence) of the phosphors used in the display. A waveform being displayed on an analog CRT has a high intensity where the slew rate of the electron beam is relatively slow and may have a lower intensity where the slew rate is relatively high. In general, this intensity variation provides useful information to a human observer. In particular, intensity provides a visual clue as to rate of change. High intensity indicates slow change and low intensity indicates rapid change. For example, if a square wave is displayed on an analog display oscilloscope, the vertical rate of change of the waveform is much higher than the horizontal sweep rate so that the high levels and the low levels of the square wave are typically at a high intensity and the rising and falling edges of the square wave are at a lower intensity.
Instruments may also use digital graphics displays similar to the displays used for most computer monitors. A digital graphics display is divided into rows and columns of addressable areas called pixels. The intensity of each pixel in a digital graphics display is typically controlled by a digital value stored in a memory associated with the display. In an instrument using a digital graphics display, a waveform is sampled, the samples are converted into digital values and the digital values are stored into an acquisition memory. The waveform samples determine which pixels are illuminated but the samples do not inherently control intensity. Any intensity variation must be accomplished as a digital computation on digital waveform data, typically by post processing the waveform image. In many systems, display update speed is a critical requirement and software post processing typically requires a relatively complex and time consuming computational burden. Alternatively, specialized hardware for high speed vector drawing may include intensity variation. A system is needed that provides intensity variation for slew rate with high speed, simplicity and low cost.